1. Technical Field of the Invention
In general, the present invention refers to a converter utilized to change the wavelength of optical signals generated by an OTDR to supervise optical networks, and a method for increasing the OTDR supervision distance range.
2. Description of Related Art
The use of optical fibers for short or long-distance digital signal transmission offers many advantages. The use of fiber is quite advantageous in relation to other transmission means such as radio, satellite, coaxial cable and twisted pairs. However, some physical effects limit optical fiber signal transmission such as, for example, the attenuation suffered by optical signals as they pass over the optical fiber. This well-known effect is mainly due to the light-scattering phenomenon known as Rayleigh scattering. The attenuation resulting from Rayleigh scattering depends mainly on the wavelength of the signal transmitted in the fiber and increases proportionally to the length of the fiber that the signal passes over.
The wavelength that least suffers attenuation when propagated over the optical fiber occurs in the optical spectrum region near 1550 nm. For this reason, this wavelength is one of the most utilized for measuring optical fiber attenuation characteristics.
The equipment utilized to supervise optical cables consists of an OTDR (Optical Time Domain Reflectometer), represented in FIG. 1, which is based on the analysis of the parcel of light backscattered by Rayleigh scattering. In the supervision process, the OTDR 1 transmits narrow pulse trains (10 μs to 10 ns), with a repetition interval on the order of a few kHz. This optical signal is generated by a semiconductor laser and transmitted to fiber 2 by means of an optical coupler 3. As the light is propagated along fiber 2, a parcel of it is being backscattered by the Rayleigh effect. This backscattering is captured by the OTDR 1, synchronized and displayed on the oscilloscope screen by means of curve A, as shown in FIG. 2, which represents the attenuation (dB) of the transmitted signal, according to the length of fiber (km) over which the signal passes. The OTDR 1 also measures the punctual reflections along the fiber. This characteristic is of fundamental importance for fault detection such as, for example, fiber break, bad splices, defective connections, etc.
OTDRs have been utilized frequently to examine the attenuation characteristics of the fiber before and after its installation as well as for quality control of fiber and optical cable at the manufacturer.
OTDRs are valuable tools for the carriers to assure preventive maintenance of installed links. The most updated optical communication systems reserve one fiber of the optical cable to be utilized by the OTDR. In this way, if a cable break occurs, it will be promptly detected and located by means of an automated supervision system that alerts maintenance teams of a defect occurrence. A limitation to this supervision method consists of the fact that there is not always a fiber available to be used exclusively by the OTDR. In such cases, the same fiber must be utilized for transmission of both normal traffic and supervision signals. However, this solution is not applicable to all optical communications systems since, in many cases, the communication signal interferes with the OTDR signal. When this happens, they must be separated by a filtering process and, in many cases, the OTDR signal must be changed to another emission spectrum band (wavelength), upper or lower to the communication signal, to allow more precise filtering.
Another limitation is related to the useful distance range of the optical supervision signal generated by the OTDR, limited to approximately 160 km of optical fiber, as shown in FIG. 2. This limitation requires one OTDR for every 160 km of supervised fiber. The supervision information is collected by the OTDRs and forwarded to the supervision center by means of the copper data network. This impedes the application of this solution in localities not serviced by the copper data network or implies in the necessity of taking the copper data network to these OTDRs. This solution significantly increases the price of the supervision system due to the need for additional OTDRs and to the expansion of the copper data network to service them.
Another issue related to the OTDRs is their elevated electric power consumption, making them unsuitable for regions without electric power supply.